With proper and careful selection of patients, fertility-preserving surgery may be feasible in patients with ovarian malignancies. However, the loss of follicles by oophorectomy and chemotherapy results in decreased ovarian reserve, which consecutively affects reproductive capacity. We evaluated postoperative levels of serum anti-Müllerian hormone (AMH) in women with ovarian malignancies to assess the impact of the fertility-preserving surgery with or without the administration of chemotherapy on ovarian reserve. In 13 patients who underwent the fertility-preserving surgery with (n = 9) or without (n = 4) the administration of chemotherapy, serum AMH levels were measured and compared with serum AMH levels measured in patients undergone cystectomy for benign ovarian tumors as a control. We found that the mean AMH level in the treatment group measured 0.9 ng/mL, which was significantly lower than that measured in the control group (4.70 ± 3.77 ng/mL). The possibility of decreased ovarian reserve occurring in patients with ovarian malignancies following treatment with fertility-preserving surgery with or without the administration of chemotherapy should be considered for fertility planning.